Removal of contaminants by filtration is commonly used and accepted practice in stormwater applications as a method for capturing fine particles. Filters employ a various array of media that capture particulate matter by bonding or capture. The media is typically granular and is contained in a device or structure that allows media compaction. This increases the media density and increases the ability of the media to remove fine particulate matter. Filters for stormwater are usually passive devices and typically fall into two categories: Horizontal filters and vertical filters.
Horizontal filters can be used in an up-flow or down-flow configuration. In a down-flow application the flow rate is driven by gravity and in an up-flow configuration the flow rate is generated through hydraulic head. In both cases, an increase in the available water column increases the flow rate. Horizontal filters tend to be larger in size than vertical filters as they achieve their surface area via a horizontal bed. Common types of horizontal filters include but are not limited to Sand Beds, Rain Gardens, and Swales. Horizontal filters can lose filtration capacity due to the plugging of the initial layer of media. Often times the media below the initial layer has sufficient capacity but the occlusion of the initial layer reduces the media lifespan. To regain filtration capacity it requires replacement or alteration of the initial or entire media bed.
Vertical filters are often times modular, which facilitates deploying them in series or parallel to achieve the desired flow rate. Vertical filters are typically located inside a containment structure that often times acts as a pre-settling device for larger particles. The vertical nature of these devices allows them to be compact and often times they are located underground. Filtration media for vertical filters is typically contained within a screened device where the vertical screen acts as the surface area for the filtration. Flow rates through the filter media are achieved via a difference in hydraulic head between the liquid in the containment vault and the filter outlet. The liquid can be directly conveyed by the pressure difference, or the hydraulic head is used to activate a siphon mechanism. Filters with siphon mechanisms typically achieve higher mean flow rates and obtain greater liquid dispersion within the media.
Vertical filters can still be subject to the plugging challenges of horizontal filters. Often times the floor of the containment structure accumulates large amounts of settled sediment. The sediment depth accrues and comes in direct contact with the filter media. When the filter is under operation during a storm event the sediment can become re-suspended in the liquid and increase the sediment loading on the filter beyond its intended design. This causes premature media failure and more frequent maintenance. Another challenge for siphon operated vertical filters is stagnant stormwater between operation cycles. A passive siphon requires a fixed hydraulic water column within the containment structure to cause siphon actuation. Variations in storm frequency and intensities can cause long periods in between operation cycles where the containment vault is partially full of stormwater but not full enough to cause siphon activation. The partial submergence of the filter media for long periods can potentially cause adverse effects and result in anaerobic conditions, organics growth, and potentially cause premature occlusion.